A new study unpacks the last 485 million years of Earth's temperature history

Climate change deniers inaccurately claim that Earth's rapidly rising temperatures are the mere product of natural cycles. Scientific experts have long explained that, although Earth does experience natural cycles, they do not explain how the average carbon dioxide level in the atmosphere is 419 parts per million, as opposed to the preindustrial level of 280 parts per million. 

Now a study in the journal “Science” provides illuminating new context about the history of Earth's temperatures — and illustrates precisely how the current spike is the result of humanity's overuse of fossil fuels.

"As a scientist, I’m always cautious of the word 'prove,' but our work demonstrates the important role that atmospheric carbon dioxide (CO2) concentrations play in controlling Earth’s temperature," Emily Joan Judd, the lead study author and paleontologist at the Smithsonian National Museum of Natural History, said to Salon.

"We know that today – and in the recent past – CO2 and temperature are directly linked; however, over really long-time scales – like hundreds of millions of years – other influences, such as the fact that the sun is getting progressively brighter, also play a role in modulating climate,” Judd continued. 

While climate change deniers seize on external variables like these to cast doubt on humanity’s culpability in the current crisis, they ignore the broader context about our planet’s history.” 

"We find a strong relationship between CO2 concentrations and average global temperatures across nearly the whole 485-million-year long record," Judd said. "This was surprising because it suggests that other factors, such as changes in Earth’s albedo (i.e., how reflective it is) or other greenhouse gas (e.g., methane) may have helped balance the impact of the increasing brightness of the sun and implies that atmospheric CO2 concentrations play an even bigger role in regulating Earth’s climate than previously thought."

Dr. Michael E. Mann — a climatologist at the University of Pennsylvania who was not involved in the study but discussed these topics in depth in his 2023 book "Our Fragile Moment” — described the new paper as "a solid study" that “makes a compelling case” for something he’s been saying for some time, “namely that we shouldn’t rule out possible ‘hothouse feedbacks’ that might lead to greater warming that many climate models predict under a scenario of climate policy inaction.” 

He continued: “That having been said, I’ve also warned that the paleoclimate record also suggests that climate models might not be capturing very well certain feedbacks [e.g. related to the 'El Nino' phenomenon] that could actually lessen the warming somewhat." 

For example, in Mann's 2021 article for the journal PNAS, he showed evidence that anthropogenic warming may cause a La Niña-like response.

“Given that, the model assimilation approach used by the authors could bias their results toward overly warm responses to past increases in greenhouse gas concentrations,” Mann said, adding he discussed such potential data assimilation approaches in this 2021 Science article. 

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While Mann remains a bit skeptical about the numbers, he said that it is certainly true that higher earth system sensitivity (ESS) is relevant to the current climate policy debate “because continued fossil fuel building will elevate global temperatures for many centuries or even millennia, wherein the slower feedbacks that contribute to ESS become more important."

Perhaps more importantly, the study demonstrates that the world has consistently been hotter when carbon dioxide levels in the atmosphere have been higher. As such, it further confirms the peril facing humanity as fossil fuel emissions continue to raise carbon dioxide levels in the atmosphere.

"These glimpses into ancient warmer worlds provide clues about what may be to come as anthropogenic emissions actively warm our planet today and into the future," Judd said. "Our work also demonstrates a strong correlation between global temperature at CO2 concentrations across the Phanerozoic, highlighting the important role of this greenhouse gas in controlling Earth’s temperature." 

The Earth today is like an icehouse, with ice sheets at both poles and comparatively lower carbon dioxide concentrations, but this has been rare rather than commonplace through the planet's history.

"This finding, in and of itself, isn’t really new — I suspect any Earth scientist who has ever taught an introductory Earth History class could tell you this — we’re just one of the first to quantify, or put numbers to it," Judd said. "It is concerning knowing that climate deniers and climate skeptics and climate delayers will inevitably point to this and say, 'See! We have nothing to worry about.' I want to stress that line of thinking is reductive, misinformed and neglects the most important aspect of the current climate crisis, which is rate: How fast CO2 and temperature change." 

However, because humans are pumping greenhouse gasses into the air at such a rapid rate, it is unlikely that most species will have enough time to adapt to the warmer temperatures.

"What is happening today is particularly problematic because, just like humans, the organisms that we share our planet with are adapted [to] the cold," Judd said. "They are not equipped to handle warmer conditions and the rate of change is too fast for evolution to keep pace." 

Similarly, humans evolved to thrive in colder societies, which is why they often establish their populations close to water sources and near sea level.

"As we observe the Earth warming at a rapid pace within human time scales, we are faced with challenges such as more frequent and intense storms, more frequent and intense droughts and floods, rising sea levels, and, ultimately, a reduction in habitable and arable land," Judd said. "Earth's resilience does not directly translate to our own ability to adapt and thrive in the face of human-caused climate change."